This invention relates to fuel cell power plants for the production of electricity from the electrochemical reaction of hydrogen and an oxidant. Preferred plants produce hydrogen from hydrocarbon fuel. In particular, this invention relates to fuel cell power plants having a device that transfers water vapor from the effluent of a fuel cell to components of the power plant that require water.
Fuel cells are devices that convert electrochemical energy from the reaction of reducing and oxidizing chemicals, into electricity. Fuel cells have been used as a power source in many applications, and can offer significant benefits over other sources of electrical energy, such as improved efficiency, reliability, durability, cost and environmental benefits. In particular, electric motors powered by fuel cells have been proposed for use in cars and other vehicles to replace internal combustion engines.
Fuel cells typically use hydrogen and air as the reducing and oxidizing materials to produce electrical energy, and water. The cell generally comprises an anode electrode and a cathode electrode separated by an electrolyte. Hydrogen is supplied to the anode electrode, and oxygen (or air) is supplied to the cathode electrode. The hydrogen gas is separated into electrons and hydrogen ions (protons) at the anode. The hydrogen ions pass through the electrolyte to the cathode; the electrons travel to the cathode through the power circuit (e.g., to a motor). At the cathode, the hydrogen ions, electrons, and oxygen then combine to form water. The reactions at the anode and cathode are facilitated by a catalyst, typically platinum.
The anode and cathode of the fuel cell are separated by an electrolyte. There are several types of fuel cells, each incorporating a different electrolyte system, and each having advantages that may make them particularly suited to given commercial applications. One type is the proton exchange membrane (PEM) fuel cell, which employs a thin polymer membrane that is permeable to protons but not electrons. PEM fuel cells, in particular, are well suited for use in vehicles, because they can provide high power and weigh less than other fuel cell systems.
The membrane in the PEM fuel cell is part of a membrane electrode assembly (MEA) having the anode on one face of the membrane, and the cathode on the opposite face. The membrane is typically made from an ion exchange resin such as a perfluoronated sulfonic acid. The MEA is sandwiched between a pair of electrically conductive elements that serve as current collectors for the anode and cathode, and contain appropriate channels and/or openings for distribution of the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts.
The anode and cathode typically comprise finely divided catalytic particles, supported on carbon particles, and admixed with a proton conductive resin. The catalytic particles are typically precious metal particles, such as platinum. Such MEAs are, accordingly, relatively expensive to manufacture and require controlled operating conditions in order to prevent degradation of the membrane and catalysts. These conditions include proper water management and humidification, and control of catalyst fouling constituents, such as carbon monoxide. Typical PEM fuel cells and MEAs are described in U.S. Pat. No. 5,272,017, Swathirajan et al., issued Dec. 21, 1993, and U.S. Pat. No. 5,316,871, Swathirajan et al., issued May 31, 1994.
The voltage from an individual cell is only about 1 volt. Accordingly, to meet the higher power requirements of vehicles and other commercial applications, several cells are combined in series. This combination is typically arranged in a “stack” surrounded by an electrically insulating frame that has passages for directing the flow of the hydrogen and oxygen (air) reactants, and the water effluent. Because the reaction of oxygen and hydrogen also produces heat, the fuel cell stack must also be cooled. Arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, Meltser et al., issued Jun. 9, 1998; and U.S. Pat. No. 6,099,484, Rock, issued Aug. 8, 2000.
For many applications, it is desirable to use a readily available hydrocarbon fuel, such as methane (natural gas), methanol, gasoline, or diesel fuel, as the source of hydrogen for the fuel cell. Liquid fuels, such as gasoline, are particularly suited for vehicular applications. Liquid fuels are relatively easy to store, and there is an existing commercial infrastructure for their supply. However, hydrocarbon fuels must be dissociated to release hydrogen gas for fueling the fuel cell. Power plant fuel processors for providing hydrogen contain one or more reactors or “reformers” wherein the fuel reacts with steam, and sometimes air, to yield reaction products comprising primarily hydrogen and carbon dioxide.
In general, there are two types of reforming systems: steam reformers, and autothermal reformers. Each system has operating characteristics that make it more or less suited to the use of particular types of fuels and in particular applications. In steam reformation, a hydrocarbon fuel (typically methane or methanol) and water (as steam) are reacted to generate hydrogen and carbon dioxide. This reaction is endothermic, requiring the addition of heat. In preferred systems, this heat is provided by a combustor that burns hydrogen that remains unreacted after the reformate passes through the fuel cell stack.
In an autothermal reformation process, a hydrocarbon fuel (typically gasoline), steam and air are supplied to a primary reactor that performs two reactions. One is a partial oxidation reaction, where air reacts with the fuel exothermally, and the other is the endothermic steam reforming reaction (as in steam reformation). The heat from the exothermic reaction is used in the endothermic reaction, minimizing the need for an external heat source.
A by-product of the reaction, in both steam and autothermal reforming, is carbon monoxide. Unfortunately, carbon monoxide will degrade the operation of the fuel cell, particularly PEM fuel cells. Thus, reactors downstream of the primary reactor are required to lower the carbon monoxide concentration in the hydrogen-rich reformate to levels tolerable in the fuel cell stack. Downstream reactors may include a water/gas shift (WGS) reactor and a preferential oxidizer (PrOx) reactor. The WGS reactor catalytically converts carbon dioxide and water to hydrogen and carbon dioxide. The PrOx reactor selectively oxidizes carbon monoxide to produce carbon dioxide, using oxygen from air as an oxidant. Control of air feed to the PrOx reactor is important to selectively oxidize carbon monoxide, while minimizing the oxidation of hydrogen to water.
Fuel cell systems that dissociate a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are well known in the art. Such systems are described in U.S. Pat. No. 6,077,620, Pettit, issued Jun. 20, 2000; European Patent Publication 977,293, Skala, et al., published Feb. 2, 2000; and U.S. Pat. No. 4,650,722, Vanderborgh, et al., issued Mar. 17, 1987.
The use of hydrocarbon reformate fuel cell systems in cars and other vehicles presents special concerns. In addition to the desirability of using readily-available liquid fuels, discussed above, the reformer and fuel cell systems must be relatively light in weight, and must be able to operate efficiently under a wide range of ambient conditions (e.g., under a range of temperatures and humidity conditions). They should also be able to be started quickly, so as to produce power within a short time interval after start-up of the vehicle. Thus, it is desirable to minimize the amount of heating of reactant components for the reformer. It is also desirable to minimize the amount of liquid water that must be handled in the system, particularly to avoid the need to replenish water within the system.
As discussed above, there are several components in the reformate fuel cell system that require water, particularly including the reformer that requires steam as a reactant, the WGS reactor, and the fuel cell that requires humidification of the MEA in order to function properly. A common approach to enhancing water balance in fuel cell systems is use of condensing heat exchangers at various points in the system. For example, heat exchangers are used downstream of the reformer to cool the reformate exhaust to a temperature at or below its dew point so as to precipitate water. The water is separated from the gaseous reformate, and stored in a reservoir. The water is then returned to the reformer where it is heated to create steam. Heat exchangers are also used to cool the exhaust stream exiting the cathode of the fuel cell so as to condense water which is used in humidifying the MEA. The use of heat exchangers presents issues, however. For example, the water recovery efficiency of heat exchangers is reduced as the ambient temperature increases. Large radiators may be required so as to dissipate the heat of condensation. Moreover, the liquid condensate produced by the heat exchangers must be vaporized for re-use in the system, creating an additional energy load and inefficiencies in the system.
Attempts to address the water balance needs in fuel cell systems have been described in the art. See, for example, German Patent Disclosure 42 01632, Strasser, published Jul. 29, 1993; U.S. Pat. No. 6,007,931, Fuller et al., issued Dec. 28, 1999; and U.S. Pat. No. 6,013,385, DuBose, issued Jan. 11, 2000. However, water management systems among those known in the art do not adequately address these needs, due to problems such as their inability to maintain true water balance over a wide range of operating conditions, mechanical complexity and reliability, increased system energy requirements, and potential safety issues.